Mechanical Properties Of Nanocrystalline Ni And Ni-Fe Alloy

Nanocrystalline material, its unique microstructure and mechanical properties,become the hot topics in the study of material science in the twenty-first century.Currently on the mechanical properties of nanocrystalline metal materials testing areconcentrated in nano-material strength, hardness, fracture toughness, compression andtensile stress-strain behavior, strain rate sensitivity, fatigue and creep. Nano-materialshave high strength and hardness, with the increase of loading strain rate, higher strength,plasticity, Its lower plasticity limit the nanocrystalline metal materials in the engineeringapplication. Nano-materials are different from the conventional coarse grained materials,its unique properties are mainly high strain rate sensitivity, inverse Hall-Petch relation，and an abnormal strain rate effect on plastic strain was observed in Nano-materials aftercompressive or tensile tests. Due to the large bulk of nanocrystalline materials aredifficult to obtain, and the quantity of materials required for the Nanoindenter is small,high precision, so widely used in mechanical properties test of nanomaterials. In theprocess of nano indentation test，the plastic deformation behavior in the holding regime isdifferent from the loading regime. This paper focused on the plastic deformation behaviorin the holding regime in oreder to fully understand the deformation mechanism ofnanocrystalline metal. More and more researchers used wide loading strain rate ofnanocrystalline metal materials in tension and compression experiment to study thestrength, plasticity and relationship between the loading strain rate. So as to enhance themechanical properties of nanocrystalline metallic materials, provide a theoretical basis.In this paper, high density and high purity of nanocrystalline Ni and Ni-25wt.%Fealloy were prepared by electrodeposition method by controlling the electric currentdensity and additives and related process parameters and improving the composition ofplating solution. The average grain size is below40nm, and grain size distribution iswider. We analysis nanocrystalline Ni and Ni-25wt.%Fe alloy composition, systemstructure and micro morphology by x-ray diffraction (XRD), scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM), and other laboratoryequipment.The nanoindentation and MTS-810tensile testing system were used to nano indentation experiments under different loading strain rate and compression test at roomtemperature with wide loading strain rate. The study results of the article are as follows:1. Preparation of the thickness of4mm, the average grain size of about37nm ofnanocrystalline Ni and thickness of600μm, the average grain size of about32nmnanocrystalline Ni-25wt.%Fe alloy were prepared by direct current electrodepositiontechnique.2. Nanoindentation tests were performed at room temperature on the nc Ni and ncNi-25wt.%Fe alloy loading strain rate from1x10-3s-1to4x10-1s-1, indentation depthsetting of2000nm, load time of500s. Through the analysis of the experimental data innanocrystalline Ni and nanocrystalline Ni-25wt.%Fe alloy, creep deformation was foundduring the holding regime. With the increase of loading strain rate, nanocrystallinematerials of creep strain and creep strain rate increases. When the indentation depth is2000nm, that the increase of loading strain rate, the indentation hardness is alsoincreasing. Calculate the value of C obleand G BSnanocrystalline Ni and Ni-25wt.%Fealloy in the holding regime, and the creep strain rate has been below loading strain rate,soas to deduce the nanocrystalline Ni and Ni-25wt.%Fe alloy creep deformation mechanismis dislocation motion.3. Through the experimental data to compare mechanical properties ofelectrodeposited nanocrystalline Ni and Ni-25wt.%Fe alloy, m of Ni is greater than that ofNi-25wt.%Fe.nanocrystalline Ni indentation hardness is less than nanocrystallineNi-25wt.%Fe indentation hardness. This is because the addition of Fe element candecrease stacking fault energy of the nanocrystalline Ni-25wt.%Fe alloy and improvepartial dislocation emission lead to the twin, such as micro twin microstructure formation.The twinning dislocations hinder expansion process can facilitate the storage ofdislocations enhance its ductility, Resulting in nanocrystalline Ni-25wt.%Fe alloy hashigher creep strain and creep strain rate than nanocrystalline Ni.4. Compression tests were performed at room temperature on the electrodepositednanocrystalline Ni with a wide loading strain rate。the compressive strength anddeformation are maximum at the loading rate for1s-1.The compression tests revealed thatnc Ni exhibits high ultimate strength3507.16MPa and good ductility of43.83%. At thecompression test, the electrodeposition of nanocrystalline Ni showed obvious workhardening behavior. Found in the experiment, a unique law of electrodeposition ofnanocrystalline Ni rate of abnormal sensitivity, with the increase of loading strain rate, the plastic has been increased. Confirmed the experimental method preparation ofnanocrystalline Ni has good strength and plasticity.